Many processes require very small particles or powders of specific materials. In the past, powders could be produced by grinding and then sieving the ground particles to produce a powder of a desired size distribution. For most applications, the material of the grinding wheel can be chosen which introduces minimal contamination. Grinding, however, has proven insufficient for some advanced applications, particularly involving fine silicon powder of very high purity level and intended for use in different phases of the fabrication of silicon integrated circuits.
Boyle et al. in U.S. patent application publication 2004/0213955 A1, now issued as U.S. Pat. No. 6,083,694, describe a recently developed adhesive bonding together silicon parts for use in the fabrication of silicon electronic integrated circuits. The silicon parts are advantageously machined from electronic grade silicon (EGS), also called virgin polysilicon, of extremely high purity so as not to contaminate the semiconductor processing with which the assembled structure is used. Virgin poly is formed by the chemical vapor deposition of silane (SiH4), trichlorosilane, or other silane compounds into generally free standing bodies. Other forms of polysilicon may be used, for example, randomly oriented polysilicon (ROPSi) grown by the Czochralski method from a randomly oriented seed. The adhesive is formed from a composite of a liquid silica-forming agent such as a spin-on glass (SOG) and fine silicon powder. After the silicon parts have been assembled with the adhesive applied to joints between the parts, the assembly is annealed at about 1000° C. to convert the silica-forming agent to silica, which apparently bonds the silicon particles to each other and to the adjacent silicon parts. It is greatly desired that the silicon powder used in the adhesive is pure enough so as to not compromise the cleanliness of the assembled silicon structure.
Silicon powder is commercially available from grinding EGS-grade silicon pellets. However, it purity level is compromised by the grinding process. Furthermore, the average particle size of the powder tends to be large, typically greater than 1 mm, and the size distribution is wide. The powder size determines the minimum clearance in the joint between parts. Generally, a small clearance and a minimum amount of adhesive in the joint are desirable. Further grinding and sieving can reduce the average size, but it becomes difficult to sieve powders below about 50 μm because of electrostatic attraction and van der Waals forces. Boyle et al. further describe the use of silicon nano-powder produced by a chemical vapor deposition (CVD) process of a vapor phase reaction of silane and hydrogen into small silicon particles of size of less than 100 nm, a size unobtainable by conventional grinding. However, it would be desirable to obtain a powder of selected size and with a narrow size distribution.